Most browsers have an option to disable EcmaScript support. (Some even put a button for this on their toolbar.) Others (such as text-based browsers) may have no EcmaScript support at all. You want your site to be viewable on any browser, don't you?

Read more at http://javascript.internet.com/page-details/no-right-click-explanation.html

why right click traps do harm

If the chosen EcmaScript snippet is smart enough to cover only images (such as the snippet below), it won't do much harm. However, most scripts disable the right-click menu everywhere, and this does more harm than good:

Some web browsers are set to open downloaded files from cache directly into their associated application and will save files to /home (or My Documents) only with a right click. An unintelligent script will make it very hard for the web designer to provide downloadables.

The "right click for the menu bar" interface in the GIMP paint program may spread to web browsers. Using a right-click trap strategy will keep them from even pressing the back button (see also back button traps).